1. Field of the Invention
This invention relates to a method and apparatus for measuring the circulatory function of a living body based on the principle of auscultation, and more particularly relates to a method and apparatus for measuring the blood pressure and pulse rate of a living body.
2. Description of the Prior Art
Methods of detecting Korotkoff sounds (hereafter referred to as "K-sounds") in a blood pressure measuring apparatus which operates based on the principle of auscultation include a method employing a filter comparator, in which the K-sounds are discriminated by using a preset threshold value, and a method relying upon pattern recognition, in which the K-sounds are discriminated based on a certain pattern.
In auscultation, also referred to as stethoscopy, a stethoscopic examination of pulse is made while gradually depressurizing a pressure cuff at a rate of 2-3 mmHg per heartbeat. The change in pressure is attended by a change in the sounds heard through the stethoscope, as shown in FIG. 1. Here A represents the point at which the K-sounds begin to be heard. The value of pressure which prevails at this point is treated as maximum, or systolic, blood pressure. The K-sounds continue to be heard clearly from point A onward and gradually increase in magnitude. At a point B, the K-sounds develop noise that continues until point C, at which the sounds return to noise-free clarity. The clear sounds continue to grow in intensity from this point until point D, where the K-sounds decrease in magnitude. The K-sounds continue losing intensity until point E, at which they vanish. The World Health Organization (WHO) recognizes point D as indicating minimum, or diastolic, blood pressure, while the Japan Insurance Association recognizes point E as indicating diastolic blood pressure.
When measuring blood pressure by auscultation, so-called "stethoscopic gaps" are sometimes encountered due to an occasional pause in the sound of blood flow during measurement. This is construed as being related to such factors as a rise in blood pressure, a decline in blood vessel resistance and the like and frequently occurs between the points B and C in FIG. 1. The phenomenon is seen in 8% of individuals afflicted with hypertension. An example of stethoscopic gaps is illustrated in FIG. 2.
(A) through (F) in FIG. 2 indicate the state of K-sound pick-up by stethoscopy when gradually reducing the pressure applied to a blood vessel. Depressurization begins with (A) and ends with (F), with the attendant change in the K-sounds being as shown. In FIG. 2, (a) through (j) indicate the occurrence of stethoscopic gap.
Noise is dealt with in the conventional blood pressure measuring apparatus in the following manner. When what appears to be a K-sound has been detected, the next K-sound should occur within a predetermined period of time. When it does not, the signal initially detected as the apparent K-sound is treated as noise. The problem encountered with this conventional approach is that if, during the course of measurement, K-sounds become so small as to be longer detectable or a change in the K-sound pattern occurs (or if a pulsating sound ascribable to the patient's pulse is produced), the K-sounds detected up to that time are construed to be noise, even though such is not actually the case. The result is that the point at which systolic blood pressure occurs is detected erroneously.
Some apparatus for blood pressure measurement come equipped with a mechanism for calculating the patient's pulse rate. In an apparatus of this type, pulse rate generally is calculated by converting the K-sound occurrence interval (or the average inverval) into the number of such occurrences per minute. However, there are occasions in the course of measurement where the amplitude of the K-sounds becomes so small as to render the K-sounds undetectable, or where there is a change in the pattern of the K-sounds, so that some K-sounds go undetected. The problem here is that if the K-sound occurrence interval at such time is used in the calculation of pulse rate, an erroneous value of pulse rate, which is lower than the true pulse rate, will be derived.
If a threshold value or pattern for distinguishing the K-sounds were to be so set as to enable diminished K-sounds or the sound of the patient's pulse to be detected in the course of measurement, a pulse sound which is produced before the K-sounds start to appear would be detected as a K-sound or, likewise, a pulse sound which occurs after the K-sounds vanish would detected as a K-sound, thus making it impossible to obtain correct systolic and diastolic blood pressure values.